Feed forward amplifiers are known in the art. In general, feed forward amplifiers separate out distortion and intermodulation components generated by a power amplifier to create an error signal. The error signal is then added to the power amplifier's output with a gain, phase, and delay adjusted for maximum cancellation of the intermodulation and distortion. The amount of distortion reduction available using feed forward technology is limited by the accuracy of gain and phase adjustments of the error signal. Prior art feed forward amplifiers have attempted to increase the accuracy of the gain and phase adjustments by injecting a test signal, or pilot, into the main signal. The test signal is then utilized to adjust the gain and phase of the error cancellation signal. Such a prior-art feed forward amplifier, generally designated 100, is shown in FIG. 1.
As shown in FIG. 1, input signal 101 is split by directional coupler 102, causing signal 101 to be directed towards main signal path 103 and error signal path 105. Test signal 107, or pilot tone 107 (generated by pilot tone generator 106), is injected via summer 109, into main signal path 103 of amplifier 100. Both input signal 101 and pilot tone 107 are amplified by main amplifier 113, and the output signal is split, with one branch input to error signal path 105, where it is subtracted from input signal 101 to produce an error signal 112. The magnitude of the pilot tone, when detected at pilot tone receiver 104, is used by pilot tone receiver 104 to adjust the gain and phase of the error signal 112 such that the addition of gain and phase adjusted error signal 114 and amplified signal 116 result in output signal 118 that has both the pilot tone and the distortion introduced by amplifier 113 substantially eliminated. In particular, gain and phase controls are adjusted by gain and phase adjuster 117 until a received signal strength indication (RSSI) of pilot tone 107 (received at pilot tone receiver 104) is minimized, causing the gain and phase of amplified signal 116 traveling through main path 103 to be properly matched with the gain and phase of adjusted error signal 114 traveling through error path 105.
The problem with utilizing a pilot tone for controlling the gain and phase of the error signal is that the addition of a pilot tone generator adds appreciable cost, board space, and isolation requirements to any feed forward amplifier. Therefore a need exists for a method and apparatus to accurately control the gain and phase of an error signal without the added cost, board space, and current drain required by prior art error cancellation methods.